Automatic telephone system



June 18,1946. G, T BAKE'Q 2,402,232

INVENTOR GEORGE THOMAS BAKER ATTORNEY June 18, 1946. G. T. BAKER 2,402,232

AUTOMATIC TELEPHONE SYSTEM Filed March 6, 1943 15 Sheets-Sheet 2 INVENTOR GEORGE THOMAS BAKER ATTORNEY June 18, 1946. -r BAKER AUTOMATIC TELEPHONE SYSTEM Filed March 6, 1 945 15 Sheets-Sheet 5 ATTORNEY June 18,1946.

G. T. BAKER AUTOMATIC TELEPHONE SYSTEM 15 Sheets-Sheet 4 Filed Mar' h 6, 1945 INVENTOR GEORGE THOMAS BAKER 4 ATTORNEY June 18, 1946. G. 1'. BAKER 2,402,232

' AUTOMATIC TELEPHONE SYSTEM Filed March e, 1945 15 Sheeis-Sheet 5 INVENTOR GEORGE THOMAS BAKER ATTORNEY June 18, 1946. G. T. BAKER AUTOMATIC TELEPHONE SYSTEM Filed March e, 1943 15 Sheets-Sheet s.

, N v e urns GORGYE THOMAS' BAQER ATTORNE? June 18, 1946.. 1}, BAK R 2,402,232

V AUTOMATIC TELEPHONE SYSTEM Filed March 6, 1945 15 Sheets-Sheet 7 'IN VENTOR GEORGE THOMAS BAKER ATTORNEY June 18, 1946. BAKER 2,402,232

AUTOMATIC TELEPHONE SYSTEM I Filed March 6, 1945 15 Shets-Sheet 8 INV E N T0 R GEORGE THOMAS BAKER ATIORNEY June 18,1946. BAKER 2,402,232

' AUTOMATIC TELEPHONE SYSTEM I Fi led March 6, 1945 15 sheets sheet 9 INVENTOR GEORGE THOMAS BAKER ATTORNEY June ,18, 1946. G. T. BAKER 2,402,232

AUTOMAII C TELEPHONE SYSTEM Filed March 6, 1943 l5.Sheets-Sheet 10 l NVENTOR GEORGE THOMAS BAKER awzzw ATTJRNEY AUTOMATIC TELEPHONE SYSTEM Filed March 6, 1943 15 Sheets-Sheet l1 INVENTOR GEORGE moms B'AKER ATTORNEY June 18, 1946.

G. T. BAKER AUTOMATIC TELEPHONE SYSTEM v Filed March 6, 1943 l5 Sheets-Sheet 12 ZZTZ GEORGE THOMAS BAKER ATTORNE Jung 18, 1946.

G. T. BAKER AUTOMATIC TELEPHONE SYS TEM Filed March 6, 1945 15 Sheets-Shet 1s INVENTOR GEORGE moms BAKER ATTORNEY June 18, 1946.

G. T. BAKER AUTOMATI C TELEPHONE SYSTEM Filed March 6, 1943 15 Sheets-Sheet 14 og 3L. v

INVENTOR GEORGE THOMAS BAKER ATTORNEY June 18, 1946. G. T. BAKER 2,402,232

AUTOMATIC TELEPHONE SYSTEM Filed March s, 1943 15 Sheets-Sheet 15 3 m w m A D =2pu1 I INVENTOR GEORGE moms BAKER ATTORNEY Patented June 18, 1946 AUTOMATIC TELEPHONE SYSTEM George Thomas Baker, Liverpool, England, as-

signor, by mesne assignments, to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application March 6, 1943, Serial No. 478,245 In Great Britain April 20, 1942 32 Claims. 1

The present invention relates to telephone systems and switching apparatus and circuits for use therewith and has for its general object the provision of a new and improved automatic telephone system in which the mechanical efliciency of power drive systems, the contact efiiciency of relay and crossbar systems and the simplicity and directness of decimal step-by-step systems have been combined to good advantage.

According to one feature of the present invention a telephone system is provided in which contact-operating members having control circuits individual thereto are driven smoothly in steps of predetermined size by a continuously operating source of mechanical power to select contact member and to cause selected contact members to engage to complete a conversational circuit free of jack, wiper or like base metal contacts. Base metal contacts are the frequent cause of microphonic noises being created in the speech circuit while the smooth running ensures the minimum of external disturbance to the contacts on which the connection depends.

According to another feature of the invention a telephone system is provided in which the incoming and outgoing leads of groups of switches are connected to co-operating contact members mounted in banks, the incoming leads of each switch being also connected to jack-in points by which contact-operating members together with controlling magnets and couplin means are adapted to be readily removed and replaced as detachable units; when in position the individual control circuits of the contact-operating members are connected via the jack-in points to the incoming leads of the switch while at the same time the coupling means are correctly aligned with respect to a rotary shaft extending along the bank. Such an arrangement enables a conversational circuit to be completed independent of a connection through the jack-in points while avoidin the necessity of using complicated common control circuits.

According to another feature of the present invention a telephone system is provided in which the speaking circuit includes a battery feed control relay set and conversational leads involving only a single pair of co-operating precious metal contacts in each lead for each of a train of switches the selection and operation of the contacts being effected by a contact-operating member whose movement is controlled by interaction between the control relay set and the individual control circuits of each switch over the leads incoming to said switch. This enables the individual control circuits of each contact-operating member and therefore of each switch to be simplified while at the same time it ensures a conversational circuit with the minimum number of contacts and therefore with the minimum number of parts which are liable to give rise to microphonic noises due to external disturbances.

According to a further feature of the present invention a telephone system is provided having banks of co-operating contact members for connecting individual sets of incoming leads of a group of switches to selected outgoing leads common to the group whereby a conversational circuit can be completed over precious metal contacts and the selectionof the co-operating contact members which are to engage can be efiected by contact-operating members which move smoothly in a step-by-step manner under control exerted over the incoming leads.

According to a further feature of the invention in a telephone system rows of conductors extend parallel to each other and cross while being normally insulated from other rows of conductors extending at right angles thereto, the said conductors having contacting points at their points of intersection; contact-operating members being controlled from a source of continuously acting mechanical power so as to move smoothly to cause conductors in one row to engage with conductors in other rows at selected points of intersection.

According to a further feature of the invention in a telephone system rows of conductors are arranged to extend parallel to each other and cross while being normally insulated from other rows of conductors extending at right angles thereto, said conductorshaving contactin points at their points of intersection, contact-operating members being controlled in a, step-by-step manner to move smoothly to cause conductors in one row toengage with conductors in other rows at selected points of intersection.

5 points of the said conductors being in juxtaposition but normally out of engagement, a contactoperating member being provided for each of said planes of contacting points which is adapted to be moved across the ends of said rows to select one of said further conductors and to move parallel to the selected conductor to engage the corresponding contacting point of a selected one of said first mentioned conductors.

A further feature of the new telephone system is especially concerned with extending the range of impulses which will effectively control the mechanisms and the setting up of a connection. It will be appreciated that in existing systems not only have the original impulse senders at subscribers premises and in other situations to generate impulses within predetermined limits, but also the impulses once generated are subject to distortion dependent upon the line conditions and also upon the number of times they are repeated and it is 'a further feature of the present invention that a telephone system is provided in which the impulse-responding mechanisms and the impulse repeaters are especially adapted to respond to a predetermined change of current in an impulse, the impulselesponding mechanisms being adapted to effect a complete operation mechanically independent of the mechanism which initiates such an operation and the impulse repeaters to generate a complete impulse independent of the device which causes the impulse to be generated.

The invention will be better understood by referring to the accompanying drawings which illustrate a telephone system by way of example embodying the features referred to above.

Referring now to the drawings,

Fig. 1 shows a perspective part view of a rectilinear continuous bank showing the metal strip multiples and the precious metal contacts, a desired connection being effected by the appropriate operation of cordless contact-operating members, conveniently referred to hereinafter as wiper arms.

Fig. 2 shows a perspective part View of the clamping and supporting arrangements for the rectilinear continuous bank of Fi 1.

Fig. 3 shows a plan view of a precious metal contacted comb plate as it is being stamped from a metal sheet which is edged with precious metal, while Fig. 4 shows how the continuous metal strip multiples are stamped from a suitable metal sheet on which precious metal contact strips have been rolled.

Fig. 5 shows a part elevational view of the rectilinear continuous bank and showing in detail an end View of part of a level, while Fig. 5a shows a corresponding plan view.

Figs. 6 and 7 together show the circuit of a ten-level ten-outlet per level group selector and the basic control circuit therefor.

Fig. 8 shows a reduced diagrammatic plan View of a cam driven two-stage selector and also the position occupied by the bank with respect thereto. Fig. 8a shows a plan View of the primary carriage release control mechanism with the carriage in its normal position, while Fig. 8b is a similar View of the conditions obtaining when the switch has completed the secondary release movement from the position to which it has been previously set and is about to commence its primary release movement.

Fig. 9 shows a part diagrammatic side elevational view of the two-stage selector and its associated bank and approximates its full size.

Fig. 10 shows a reduced perspective view of the selector chassis with the selector components assembled thereon.

Fig, 11 shows the slight mechanical modifica- 4 tions which are involved in meeting special release action requirements.

Fig. 12 shows an enlarged end view of one end of one leg of a carriage shown on its running rail, while Fig. 13 shows an enlarged perspective view of the primary and secondary carriages together with the wiper arms.

Fig. 14 shows an enlarged sectional view of the primary drive main and subsidiary cams, which, as shown, are integral with one another, while Fig. 15 ShOWs an enlarged perspective view of the selector secondary drive arrangements from which, for the sake of clarity, the main drive cam is assumed to have been removed. The four basic stages of operation involved in driving the selector primary carriage one step is shown in the four figures 16 to 160 inclusive.

Fig. 17 shows a reduced perspective view of the complete selector jacked in position adjacent the bank.

Fig. 18 shows a front elevation of exchange mounting uprights, one of which is equipped with two twenty-selector bank multiples and associated selectors, another with two twenty-selector bank multiples only, while the remainder are shown in various stages of assembly.

Fig. 19 shows circuits of a 10/10 battery testing group selector.

Fig. 20 shows circuits of a 10/10 regular final selector and Fig. 21 shows circuits of a 2-10 line P. B. X 10/10 final selector.

Fig. 22 shows circuits of the control relay set for a train of group selector and final selector switches.

Fig. 23 shows circuits of the switching arrangements for outgoing selector levels.

Fig. 24 shows basic circuits of a D. C./A. C. outgoing impulse repeater for inter-exchange working, while, Fig. 25 shows similar circuits of a relay set for terminating the incoming end of inter-exchange junctions.

As so much of the new telephone system according to the present invention is concerned with a special form of contact bank, a description will first be given of the construction of a preferred form of bank according to the invention.

A bank is formed by continuous strips 23 running the full length of the predetermined multiple and as shown in the part view of Fig. 1 and in Figs. 5 and So. these are clamped at opposite sides of selectors to form fixed contact members the intervening space between the clampings forming the bank contacts for one selector. At the end of the multiple it will be understood that the strips 23 are fanned out to form wirin tags. That part of the bank multiple allocated per selector will subsequently be referred to as a selector bank so that Fig. 1 may be described as showing a part view of two selector banks in each of which are included three levels of two conductors each. A level of one bank is moreover assumed to have been broken away in order more closely to illustrate its construction. Each bank will be preferably of the point type, that is to say, with ten levels of ten sets of contacts, each set of four contacts being formed on four strips and four combs 24; on each level there will be provided in addition the usual eleventh step set of contacts 25 for overflow busy indication'purposes.

A multiple will usually comprise twenty such banks assembled together one above the other, the necessary 'multipling being inherently performed by virtue of the use of continuous strips 23 which connect with the outgoing trunks. The

connecting function of the ordinary selector wipers and cords is taken by a set of comb plates 24 one of which is provided for each row of strips, conveniently referred to as a level, of each bank and having fingers 26 aligned with the various bank strips. Each finger of a comb plate such as 24 forms a movable contact member. All bank contacts are made of precious metal such as silver, and to make this a commercial proposition all the silver/base metal junctions are made by rolling, this rolling operation being carried out in the stock before blanking.

The continuous strips are stamped from a metal sheet such as In (Fig. 4) the width of which sheet equals the length of strip required. On this sheet silver contact strips will have been previously rolled at right angles to the direction of the trunk strips (as shown) and at spacings of say 1 /2 inches, this distance representing the calculated vertical distance required between bank centres to accommodate the selector switches one above another. The comb plates are stamped from springy metal ribbon H (Fig. 3) of suitable width and provided with a rolled silver edge of, say, about 20 mils width. The comb plate fingers are preferably each split with a saw out (not shown) to give a twin contact effect and the plates are subsequently formed to give the requisite shape shown in Figs. 1 and 5.

Each row of contact members in a bank is separated from its neighbour by an insulating plate such as l2 (Figs. 5 and 5a) of a length equal to that of the comb plates and of a thickness of say 50 mils to reduce possible cross talk to a minimum and at the same time to give ample room in the level for the insertion of the wiper arms. Since each trunk strip will be clamped in each of the twenty banks through which they pass, it is not considered necessary to provide clamping bars which extend through the banks, and accordingly it will be seen that each bank of the multiple after assembly in a suitable jig will be clamped together by two horizontal clamp plates 13 and [4 (Figs. 2, 5 and 5a) which are secured against the bank by clamp rods and IS. The requisite spacing of 1% inches between adjacent banks is secured by means of the four vertical members ll, [8, l9 and which extend over the length of the multiple and which are secured at each bank by the two clamp rods provided individual thereto.

As will be described later each selector switch mechanism is adapted to jack into position against its bank in the manner shown in Fig. 17, with its wiper arms normally at rest at a distance equal to one level before the first level of the bank. To meet this requirement suitable packing pieces 2| and 22 (Fig. 8) are interposed between vertical bars l9 and 20 and the adjacent clamp bar [4. From Fig. 8 it will 'be seen that the clamp rod IE will be nearest to the selector switch mechanism and this will be of such a length as to finish off level with the vertica1members l1 and [9 including its fixing nuts, or alternatively countersunk fixing screws may be used as in Fig. 17. The other clamp rod IE will be of such a length as to extend for about of an inch at one end in order to provide suitable support for the bank multiple on the mounting upright.

Owing to the fact that the new selector requires considerably less space than existing ones the cabling should receive special consideration. Cabling will conveniently be carried at three levels, say 4 0"; I 3"; l0 6" from the floor,

by runways 9|, 92 and 93 respectively. The heights correspond to the top of the three 20 point'vertical bank multiples such as 90, and accordingly the wiring from the multiple tags can drop straight into the runway. The incoming wires either rise to the runway above or'drop to the one below dependent on their final destination. Thus most of the exchange wiring is already terminating virtually in a runway and the remainder reaches it by a very direct route.

It should be noted that each upright will accommodate sixty selectors, such as 96. Assuming that the selectors are constructed as previously described with a common driving shaft for a group of switches, then the uprights such as 94 and 95 may also serve to carry and align the main driving shafts such as 61.

The main driving shaft 91 is divided up into three sections, one per multiple (it being assumed that the top section has not been fitted). The sections are connected by a flexible coupling such as 99; the bottom one connecting direct with the motor l 813. 'Each section has three bearings such as H31, H12 and IE3 bolted on to extension arms on the upright, the upper one also containing a thrust bearing to take the weight of the shaft. The extension arms carry the bearings on the side away from the selector, the other side being a flush fit against and supporting the vertical members of the bank. The projecting end of all the rear bank clamp rods are bolted into the angle-iron upright. Th position of the nut on each clamp rod determines the relation between the multiple and the shafting and accordingly as each rod is bolted up a gauge corresponding to the selector will be slid into the selector mountingand the nuts adjusted to ive the correct clearance.

It will be seen that the methods adopted allow a very flexible method of extending or equipping an exchange. Suppose only one multiple is required on an upright. The angle iron serves the three multiples on th same upright. All the other items however are put in for one multiple only, i. e. the extension arms, bearings, shaft and bank. Should another multiple be required, the bearing shaft and bank, etc., can be added without disturbing the existing equipment. This makes it possible to fill up the uprights completely without leaving spare multiples.

The actual arrangement of selectors on the various uprights is such as to give the minimum cabling. Two methods are possible, one, in which all the first selectors occupy certain uprights, second selectors other, and final selectors still more. The second method is that in which first selectors occupy the top multiple on all uprights, second selectors the centre one and final selectors the bottom or vice versa. The second arrangement by spreading a single rank of selectors over a number of shafts, makes the eiTect of a drive failure of one shaft less important.

In the event of a single motor failure, sprockets of the bicycle type may be fitted on th shaft having the faulty drive and on an adjacent one and connected by a length of bicycle chain allowing one motor to take the dual load whil repairs were being effected.

If motors are provided common to an upright from an exchange maintenance point of view a further improvement might be incorporated in the switch construction which would further enhance its value. Briefly, the switch cover could form an integral part of the switch and within which a short individual roller drive would be located. To this drive would be coupled a pinionwhich would slightly project; from the switch in such manner as to engage with a corresponding pinion on a common motor driven driving shat-t when th switch is jacked into position. The short roller drive within each switch would thus be in continuous rotation so long as the switch is jacked in position and would be coupled up by the friction driving cams in the same manner as coupling is effected with the common driving roller. The advantage obtained is that each switch could be a sealed unit which would be sold to a customer as such, and which, if anything went wrong, would be returned to the manufacturer for repair or reeadjustment where the best possible arrangements for adjustment and/ or repair would be available. This feature would have particular appeal in the case of private automatic exchanges where skilled maintenance personnel and suitable maintenance equipment are not ordinarily readily available. The sealed cover arrangements would furthermore provide an eiiective remedy against unnecessary wear and trouble which results from th ingress of dust and abrasives which is always likely to occur when maintenance and equipment is performed on site, as is usual at present.

The switch mechanism which controls the movement of the wiper or contact-operating arms has two movements, one across the contact bank and the other into the contact bank. These movements will be referred to as primary and secondary movements respectively. A description of the switching construction will now be given with particular reference to Fig. 8.

With regard to Fig. 8 it should be explained that it is partly diagrammatic and not in exact accordance with th exact construction. For instance. the driving cams are shown circular with a central pivot and overlapping the continuously operating roller, whereas the primary cam should be of the shape shown in Fig. 16 and should be pivoted. eccentrically as shown. The secondary driving cam which is adapted to advance the secondary carriage through two steps per revolution is of the same size as the primary cam, but the shape of both this secondary cam and its subsidiary cam are modified accordingly, the shape of the subsidiary cam being on the lines indi ated in the perspective view of Fig. 15.

A further point by which the selector shown in Fig. 8 difiers from actual practice is that the primary cam is shown to be of the two-stage escapement release type while the secondary cam is shown to be of the single stage simple release type so that the former is only released and allowed to engage with the driving roller when the corresponding controlling relay releases after having been operated, while the latter is released and allowed to engage with the roller as soon as the secondary control relay operates. With the simple circuit arrangements to be described the selector is therefore only suitable for functioning in a group selecting capacity when it has to respond to trains of impulses for its primary direction of movement and when it has to self -drive in the secondary direction so as to perform a hunting operation. Both primary and secondary cams may he of the simple catch release type shown in connection with the secondary cam in Fig. 8 if the circuit arrangements are suitably modified.

Referring to Fig. 8 it will be seen that the power source for the operation of the selector is derived from a hard rubber roller 41. rotating on a comon sha at a n ta t sp ed. ay 10 1?- The p m r and c n ary dri ing c ms .8 an 43 are each eccentric and pivoted so as to be just c ar h ro l on. ts smal t a us Each cam has a spring 12 or 64; associated with it which s res. fiic n er to e he cam a Small rotational movement when a catch controlled by a control relay '14 or 15 is released. When a control relay has efiectively released the catch the surface of the cam after a slight movement comes n o on act wit he r r whic continues t e rotation of the cam. As is later fully described, the pivot of the cam is moved away from the roller as the cam rotates, the pressure between the cam and the roller due to the load automatically preenting slipp n h pivo f the a transmits its movement by a single member [6 or 60 to direct a carriage 59 or 52, A subsidiary cam int a wi h he mai c m a operating upon a fire surface caus h driv pin 8 t be moved in and out of engagement with the driving teeth thus avoiding ratchet and pawls and ensuring a constant lock of the carriage at all times. The contour of the main cam is such as to give constant acceleration and deceleration to the carrieee thus reducing stresses to a. minimum.

The switch is built up upon a chassis 3| on whic h prima y an s n a c n l re ays It, 15 are mounted, either directly or indirectly as will be later described. Two carriages are provided, a U-shaped primary carriage 52 (see Fig. 13), the two side arms 53 and 54 of which form the ser arm a an L- h ped se ndary carriage 58, on the long arm 5! of which the primary carriage is adapted to run. Each of the carriages runs on rollers and is provided with a toothed driving member, the primary carriage being adapted to be moved step-by-step on a rail pro- Vided by the long arm of the secondary carriage a d th t car t t the i r a m 53 and 54, a state of balance therefor being maintained by the wiper arms which rest on a rail 34 positioned at right angles thereto and forming an extension of the release magnet armature 32 (see also Fig. 10). The secondary carriage is adapted to be moved step-by-step onrails provided by the chassis and thus to carry with it the primary carriage and associated wiper arms.

As an introduction to the operation of the switch it l he assumed h t i is f nct oni as a final selector, i. e. both basic movements are controlled by dialled impulses.

In this case both primary and secondary cams will have to be of the two-stage release type or alternatively suitable circuit arrangements will have to be provided to allow of impulse control of single-stage release cams.

On receipt of the first train of impulses, the primary relay 14 responds and in a manner to be later described intermittently couples the associated driving cam 48 with the roller drive 41 with the result that a driving member 16 pivoted 0n the same pivot as the cam is given a reciprocating and rocking motion and its tip co-operates with the toothed driving member of the primary carriage 5 2 to advance it a number of steps corresponding to the number of impulses dialled and thereupon to set the wiper arms to a position opposite the required bani; level. The release magnet 18 is then energised, whereupon rail 34 secured to the magnet armature 32 raises the wiper arms 53 and 54 slightly at their tips so as to cause these tips to clear the toothed combs 24 of the bank level into which they will be subsequently inserted. The ba con is o e c tact m e s 23 extending from one side of the switch to the opposite side in the form of metallic strips and movable contact members, one for each fixed contact member in each switch which form the teeth of a comb 24, insulators l2 being provided between the comb plates and strips as illustrated in Fig. 1. The second train of received impulses is effective on the secondary relay 15 which in response thereto intermittently couples the associated driving cam 49 with the roller drive and the secondary driving member 60 is also given a reciprocating and rocking motion so as to cause the secondary carriage 50 to move a number of steps corresponding to the digit concerned and in so doing to carry with it the primary carriage and also the wiper arms mounted thereon. At the end of the second digit the wiper arm tips will be positioned over the required set of contacts in the bank and the release magnet is now de-energized to cause the tips to lower into the contact sets opposite to which they are positioned in order to make firm contact between the incoming trunk and the selected outgoing trunk.

When the switch is to be released the release magnet is again energised and it will be understood that the mechanical conditions of the primary and secondary ratchet retaining pawls are made such as to allow the carriages to restore to their normal positions under local spring tension, the wipers travelling light as during the initial setting operation 'of the switch, since the wipers wfll have again been lifted clear of the banks on the energisation of the release magnet.

In the development of the new automatic telephone system in which the selector according to the present invention forms a large part, mechanical and electrical design were both taken into consideration, and at this stage a preliminary circuit review will be introduced since some of the circuit aspects of the system had a very large influence on the final design.

It will, however, be appreciated that certain of the mechanical features are capable of being used independently of the circuit design and vice versa, the present invention aiming at showing how the two features of design may be com bined with advantage.

One of the major distinctions of this selector switch is that the connecting function is now part of the bank and virtually divorced from the selector. It seems highly desirable to maintain this condition rigidly not only for selector simplification but also for talking efificiency. Hence all switching contacts must be kept out of the through wires which in every selector must look i like the right-hand top portion of Fig. 7 or the top portion of Fig. 19, from which it will be seen that the speaking path at each selector passes through neither relay contacts nor jack-in plug and socket points.

In designing circuits to meet this condition the following guiding factors were taken into consideration:

1. It is extremely advisable'that the holding and battery feeding bridge shall be in the first stage or at least in an early position in the train.

2. The avoidance of line contacts means that selectors provided with two or more sets of wiper arms and banks should not be provided as these switching relays so that generally there will be only one available outlet per step on any level such as on a outlet (ten levels of ten outlets each) selector and it is proposed to usesuch a 100 outlet selector in the new system. The new selector drive arrangements may, however, enable a secondary movement search speed to be obtained, which will permit of fifteen or more outlets to be hunted over in a direct line. The use of such selectors also permits four wires to be used and this fourth wire is made use of to simplify subsequent circuit design. It also simplifies the circuit problems which arise when a feeding bridge is employed at any early stage in the switch train.

3. The use of the fourth wire opens up the possibility of avoiding the use of independent relays in the selectors. The primary and secondary magnets may be used as relays, since they can conveniently be of the same general design as the standard British post-office type relays.

4. The necessity of eliminating an extra mounting means for relays enables the selector to assume whatever shape is most suitable for its own purpose and it is found that a fiat type of construction with a height only of the order of 1% inches is quite possible.

Every selector normally contains three basic relays usually designated A, B and C. The present circuits are such that these relays need be provided only once per train thus further justifying the use of 100 outlet selectors and effecting material economies in relays.

One of the problems created by employing the battery feeding relays at an early stage in the connection is that of impulsing. In this case the relays usually subject to impulse failure are concentrated in the first stage so no difference is encountered there. The magnets are subject to an initial extra repetition due to this cause, but in any case, this is met by the elimination of a repetition in the selectors themselves where the incoming impulses are delivered direct into the magnets instead of being repeated from an A on one pulse however long or short.

changeover by the operation of the combined release magnet and relay ZR. This unit is of comparatively high resistance and hence little battery drain is involved'in maintaining it energised until the completion of the secondary movement. Immediately this motion commences relay VR, which also serves as the switch holding relay, is maintained from the private and on its release at the end of the call re-energises the release magnet to initiate release of the switch. t

The only controls to reach the selector come over the four incoming wires. All tones and common services are supplied at the battery feeding relays. Accordingly it would appear that only battery and earth connections are required in addition to the four control wires. Hencea six point test jack has been used in place of the usual U point jack. Where selectors have to fulfil some special function this number wouldundoubtedly be increased but if the selector is definitely kept solely to its mechanical functions the number required is quite small.

A detailed description of these circuits will be made later after consideration has been given to the selector switch mechanism, for which reference should be had firstly to the plan layout thereof shown in Fig. 8. From this figure it will be noted that it has been possible to accommodate a two-coil release magnet T8 so that ample 

